diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index 39dde23b28a55..9d2566b7b0b05 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -3468,6 +3468,38 @@ namespace ts.projectSystem {
         it("works when project root is used with case-insensitive system", () => {
             verifyOpenFileWorks(/*useCaseSensitiveFileNames*/ false);
         });
+
+        it("uses existing project even if project refresh is pending", () => {
+            const projectFolder = "/user/someuser/projects/myproject";
+            const aFile: FileOrFolder = {
+                path: `${projectFolder}/src/a.ts`,
+                content: "export const x = 0;"
+            };
+            const configFile: FileOrFolder = {
+                path: `${projectFolder}/tsconfig.json`,
+                content: "{}"
+            };
+            const files = [aFile, configFile, libFile];
+            const host = createServerHost(files);
+            const service = createProjectService(host);
+            service.openClientFile(aFile.path, /*fileContent*/ undefined, ScriptKind.TS, projectFolder);
+            verifyProject();
+
+            const bFile: FileOrFolder = {
+                path: `${projectFolder}/src/b.ts`,
+                content: `export {}; declare module "./a" {  export const y: number; }`
+            };
+            files.push(bFile);
+            host.reloadFS(files);
+            service.openClientFile(bFile.path, /*fileContent*/ undefined, ScriptKind.TS, projectFolder);
+            verifyProject();
+
+            function verifyProject() {
+                assert.isDefined(service.configuredProjects.get(configFile.path));
+                const project = service.configuredProjects.get(configFile.path);
+                checkProjectActualFiles(project, files.map(f => f.path));
+            }
+        });
     });
 
     describe("tsserverProjectSystem Language service", () => {
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index a5b09c6a63f5d..c4446c049b6ec 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -725,15 +725,6 @@ namespace ts.server {
             }
         }
 
-        private findContainingExternalProject(fileName: NormalizedPath): ExternalProject {
-            for (const proj of this.externalProjects) {
-                if (proj.containsFile(fileName)) {
-                    return proj;
-                }
-            }
-            return undefined;
-        }
-
         getFormatCodeOptions(file?: NormalizedPath) {
             let formatCodeSettings: FormatCodeSettings;
             if (file) {
@@ -1991,13 +1982,24 @@ namespace ts.server {
             return this.openClientFileWithNormalizedPath(toNormalizedPath(fileName), fileContent, scriptKind, /*hasMixedContent*/ false, projectRootPath ? toNormalizedPath(projectRootPath) : undefined);
         }
 
+        private findExternalProjetContainingOpenScriptInfo(info: ScriptInfo): ExternalProject {
+            for (const proj of this.externalProjects) {
+                // Ensure project structure is uptodate to check if info is present in external project
+                proj.updateGraph();
+                if (proj.containsScriptInfo(info)) {
+                    return proj;
+                }
+            }
+            return undefined;
+        }
+
         openClientFileWithNormalizedPath(fileName: NormalizedPath, fileContent?: string, scriptKind?: ScriptKind, hasMixedContent?: boolean, projectRootPath?: NormalizedPath): OpenConfiguredProjectResult {
             let configFileName: NormalizedPath;
             let sendConfigFileDiagEvent = false;
             let configFileErrors: ReadonlyArray<Diagnostic>;
 
             const info = this.getOrCreateScriptInfoOpenedByClientForNormalizedPath(fileName, projectRootPath ? this.getNormalizedAbsolutePath(projectRootPath) : this.currentDirectory, fileContent, scriptKind, hasMixedContent);
-            let project: ConfiguredProject | ExternalProject = this.findContainingExternalProject(fileName);
+            let project: ConfiguredProject | ExternalProject = this.findExternalProjetContainingOpenScriptInfo(info);
             if (!project) {
                 configFileName = this.getConfigFileNameForFile(info, projectRootPath);
                 if (configFileName) {
@@ -2007,6 +2009,10 @@ namespace ts.server {
                         // Send the event only if the project got created as part of this open request
                         sendConfigFileDiagEvent = true;
                     }
+                    else {
+                        // Ensure project is ready to check if it contains opened script info
+                        project.updateGraph();
+                    }
                 }
             }
             if (project && !project.languageServiceEnabled) {
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index abd03c26b0b2a..8ae40d84afd97 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -7605,7 +7605,6 @@ declare namespace ts.server {
          * @param forceInferredProjectsRefresh when true updates the inferred projects even if there is no pending work to update the files/project structures
          */
         private ensureProjectStructuresUptoDate(forceInferredProjectsRefresh?);
-        private findContainingExternalProject(fileName);
         getFormatCodeOptions(file?: NormalizedPath): FormatCodeSettings;
         private updateProjectGraphs(projects);
         private onSourceFileChanged(fileName, eventKind);
@@ -7723,6 +7722,7 @@ declare namespace ts.server {
          * @param fileContent is a known version of the file content that is more up to date than the one on disk
          */
         openClientFile(fileName: string, fileContent?: string, scriptKind?: ScriptKind, projectRootPath?: string): OpenConfiguredProjectResult;
+        private findExternalProjetContainingOpenScriptInfo(info);
         openClientFileWithNormalizedPath(fileName: NormalizedPath, fileContent?: string, scriptKind?: ScriptKind, hasMixedContent?: boolean, projectRootPath?: NormalizedPath): OpenConfiguredProjectResult;
         /**
          * Close file whose contents is managed by the client